Throughout this application, various publications are referenced by author and date. Full citations for these publications may be found listed alphabetically at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed herein.
The insight that memory has time-dependent phases dates to 1890 when William James first proposed a distinction between a primary or short-term memory, a memory that has to be maintained continuously in consciousness, and secondary or long-term memory that can be dropped from consciousness and could be recalled at will at a later time (James, 1890). According to James view, short-term memory holds information for a few seconds whereas long-term memory holds information for long periods of time. Subsequent experimental work suggested that these two phases of memory are usually in series and that the transition from short- to long-term memory is facilitated by an increase in the saliency or the number of training trials (Ebbinghaus, 1885; Weiskrantz, 1970; Craik and Lockhart, 1972; Wickelgren, 1983; Mandel et al., 1989).
The distinction between these two major phases was placed on a firmer biochemical basis when long-term memory was found to require the synthesis of new proteins, whereas short-term memory does not (Davis and Squire, 1984). These Biochemical studies also revealed that short-term memory often lasted many minutes, and therefore was more enduring than the primary memory delineated y James. These studies therefore suggested that short-term memory may in turn have subdivisions, and that in addition to primary or working memory, there is a subsequent intermediate stage of, protein synthesis-independent, short-term memory. Further support for subcomponents of memory have also emerged from genetic studies in Drosophila and pharmacological studies in rodents and chicks (McGaugh, 1968; Cherkin, 1969; Gibbs and Ng, 1977; Frieder and Allweis, 1982; Rosenzweig et al., 1993; Tully et al., 1994; Zhao et al., 1995 a and b; Bennet et al., 1996).
In addition to being able to distinguish temporal phases in memory storage, studies in human and monkey also delineated two distinct neural systems for long-term memory based upon the types of information stored. Bilateral lesions of the medial temporal lobe revealed an impairment in declarative long-term memory, a memory for people, places and objects but these lesions spared non-declarative memory for perceptual and motor skill. Particularly, interesting was the finding that the lesions of the medial temporal lobe system, that interfere with declarative memory, only interfere with the long-term form of this memory and not with components of short-term memory, in particular not with working memory (Scoville and Milner, 1957; Mishkin, 1978; Zola-Morgan and Squire, 1985; Squire, 1987; Overman et al., 1990; Alvarez et al., 1994). These results indicate that structures in the medial temporal lobe, in particular the hippocampus, specifically subserve long-term memory but not some components of short-term memory.